In recent years consideration is being given to the adoption of multiplexed transmission systems for relay channels (mobile backhaul) connecting a base station and a backbone circuit. In particular, attention is being paid to applying radio communication (radio communication by OAM system) using an electromagnetic wave having OAM, to the abovementioned relay channels.
An OAM system, as shown in NPLs (Non-Patent Literatures) 1 to 4, is a multiplexed transmission system that uses orthogonality of phase in electromagnetic wave propagation direction vertical plane, and individual electromagnetic waves that are orthogonal are called modes. In the OAM system, by using the orthogonality of the modes in question, multiplexed transmission is realized. More specifically, a transmission apparatus according to the OAM system generates a plurality of modes when signals (data) are transmitted, and combines these for transmission. Meanwhile a receiving apparatus extracts signals by separating the combined modes in response to each mode, to realize multiplexed transmission.
In the OAM system, it is possible to distinguish between a mode for transmitting signals (below, denoted as transmission mode) and a mode for receiving signals (below, denoted as receiving mode). The transmission mode corresponds to a mode in which a post-modulation signal sequence is converted to an electromagnetic wave having orthogonality in the OAM system on a transmission side. The receiving mode corresponds to a mode for separating the original signal sequence from an electromagnetic wave having orthogonality in the OAM system on a receiving side.
PTL (Patent Literature) 1 discloses technology that performs correction of antenna axis using phase difference information.
[PTL 1]
Japanese Patent Kokai Publication No. JP2015-231108A.
[NPL 1]
L. Allen, M. W. Beijersbergen, R. J. C. Spreeuw, and J. P. Woerdman, “Orbital angular momentum of light and the transformation of Laguerre-Gaussian laser modes,”Phys. Rev. A, vol. 45, no. 11, 1992.
[NPL 2]
M. V. Vasnetsov, V. A. Pas'ko, and M. S. Soskin, “Analysis of orbital angular momentum of a misaligned optical beam,”New J. Phys. vol. 7, No. 46, 1-17, 2005.
[NPL 3]
Y.D. Liu, et al., “Orbital angular momentum (OAM) spectrum correction in free space optical communication”, Opt. Express Vol. 16, No. 10, 2008.
[NPL 4]
Y. Zhou, H. Tian, G. Nie, “Novel Method of Axis Alignment in Orbital Angular Momentum Wireless Communication”, IEEE WCNC 2015, Track1, 586-590, 2015.